DESCRIPTION: Conductive hearing losses result from a wide range of pathological conditions, but all degrade people's ability to communicate. Modern surgical practice includes dramatic procedures for treatment of conductive hearing problems, including reconstruction of abnormal or diseased external and middle ears, but the effects of some treatments on the patients hearing are often variable and hard to predict. Choices among medical and surgical treatments are guided by diagnostic tests, by physicians' experience and by theories of how pathologies affect the normal processes of the external and middle ear. However, many current theories are based on anecdotal evidence, incompletely tested ideas, or poorly controlled observations. The ears of animals provide an opportunity to determine unmistakably the effects of structural variations on the ear's function, and to develop and test theories of the ear's operation. These structures of the external and middle ear that control the collection and delivery of sound signals from the environment to the inner ear, and thence to the brain, vary in their configuration among species as well as during development from newborn to adult, and controlled experimental manipulation is possible in physiological experiments on anesthetized animals. We use all these approaches to determine the effects on hearing of variations of external- and middle-ear structures. Specifically, measurements of middle and external-ear structure and function will be made in anesthetized rodents with different middle-ear structures and in developing cats. The proposed functional and structural measurements will lead to a quantitative theory of how the external and middle ear function together. One result will be a quantitative understanding of how hearing is affected by the wide variations in the air volume of the normal and pathological human middle-ear cavities. One benefit of this knowledge will be that acoustic effects of fluid in the middle ear (an abnormal condition which occurs frequently in children) will be explained and thereby both inform physicians' judgments on causes of children's hearing loss and make possible prediction of the effects of treatments. Our expected results may also explain deficits in people's ability to identify the direction from which sound arrives, as a result of abnormal ear structures. A listener's ability to "localize" talkers, in perceptual space, is crucial to communication in environments where more than one person speaks simultaneously, such as dinner tables and bus stations.